Thyroid cancers are associated with a high prevalence of non-overlapping mutations of the genes encoding the receptor tyrosine kinases RET and TRK, as well as of NRAS, HRAS, KRAS and BRAF. Mutations of the RAS family are commonly associated with follicular thyroid cancer (FTC), while mutations in BRAF are the most prevalent genetic defect in papillary thyroid cancer (PTC). A common property of these proteins is their ability to signal via the classical MAPK pathway, which has been interpreted as evidence for a critical role of MAPK in initiation and maintenance of thyroid cancer. However thyroid cancers are also associated with mutations of genes encoding effectors signaling along the phosphatidylinositol 3-kinase (PI3K) pathway, yet whether constitutive activation of PI3K-AKT cooperates or not in certain thyroid tumors with MAPK to drive the tumorigenic process has not been carefully explored. Ras oncoproteins signal through both pathways, whereas oncogenic Braf signals preferentially (or exclusively) through MAPK, providing a useful paradigm in which to explore these interactions in mouse cancer models. The intensity of the oncogenic signal is an important variable, and we intend to use genetic manipulations that approximate pathophysiological conditions in these analyses. For this we have generated mice with a conditional gene- targeted activating mutation of Hras, and obtained an analogous model for Braf, which after Cre-mediated recombination result in expression of the respective oncoprotein under the control of their endogenous gene promoters, hence providing a physiological stimulus for tumor initiation. This proposal we will explore whether Hras or Braf activation cooperate with loss of the tumor suppressor Pten to promote thyroid cancer progression. We hypothesize that these oncogenes cooperate with loss of the tumor suppressor Pten based on information from human thyroid cancer samples, and preliminary data from our mouse genetic studies. The following aims will be pursued: I. Determine whether targeted knock-in of HrasG12V cooperates with thyroid-specific Pten loss to induce thyroid cell growth and transformation to follicular thyroid cancer. II. Examine whether thyroid-specific loss of Pten increases the virulence of papillary thyroid tumors in mice expressing endogenous levels of oncogenic Braf. The goals of this research are to better understand the genetic events that lead to thyroid cancer development and to establish relevant models of thyroid cancer to develop new treatments for this disease.